Lateral patellar instability is almost always due to a patellar (kneecap) dislocation. In this circumstance, an athlete or patient will experience their kneecap slipping out of the lateral (outside) aspect of their knee. It almost always occurs with the knee straight or at shallow degrees of knee flexion rather than with the knee bent. When the patella dislocates, it tears the structures on the inside of the knee, with the medial patellofemoral ligament (MPFL) most commonly torn.
Pain is the predominant symptom and is usually gradual in onset. Patients may experience a dull aching pain around the sides, below or behind the kneecap. Sometimes, climbing stairs and standing up or walking after prolonged sitting may produce a popping or cracking sound in the knee. The pain may also be present at night and be exaggerated by any repetitive knee bending activity such as jumping, squatting, running or weight lifting. Any changes in the activity level, playing surface or equipment may also result in pain.
Pain in kneecap with activity and when palpated
Feeling of kneecap slipping with twisting or turning movements
In rare cases a near knee dislocation (subluxation) when the knee is close to being straight
Patellofemoral instability can be caused because of variations in the shape of the patella or its trochlear groove as the knee bends and straightens. Normally, the patella moves up and down within the trochlear groove when the knee is bent or straightened. Patellofemoral instability occurs when the patella moves either partially (subluxation) or completely (dislocation) out of the trochlear groove.
A combination of factors can cause this abnormal tracking and include the following:
Anatomical defect: Flat feet or fallen arches and congenital abnormalities in the shape of the patella bone can cause misalignment of the knee joint.
Abnormal Q angle: The high Q angle (angle between the hips and knees) often results in mal tracking of the patella such as in patients with knock-knees.
Patellofemoral arthritis: Patellofemoral arthritis occurs when there is a loss of the articular cartilage on the back of the kneecap. This can eventually lead to abnormal tracking of the patella.
Improper muscle balance: Weak quadriceps (anterior thigh muscles) can lead to abnormal tracking of the patella, causing it to subluxate or dislocate.
Young, active individuals involved in sports activities are more prone to patellofemoral instability.
There are many factors to evaluate in a patient’s prognosis when a lateral patellar dislocation exists. The cause of lateral patellar instability is very important to plan the proper treatment, and each patient should be evaluated in terms of history, physical exam, X-rays, and MRI scans to determine the best treatment for them.
In the majority of circumstances, we treat these without surgery and attempt rehabilitation. Patients, who have a normal patellar height and when injured, do not knock off any pieces of articular cartilage or bone, have a fairly good prognosis and have a low risk of recurrent patellar instability.
Patients who have recurrent instability may need surgery to restore both the medial stabilizing ligaments of the knee and any other associated pathology. This can include a medial patellofemoral ligament reconstruction, a lateral retinacular lengthening, a tibial tubercle osteotomy, and/or a trochleoplasty.
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